1. Field of the Invention
The invention pertains to photopolymerizable compositions and photographic elements containing such photopolymerizable compositions. More particularly, the invention pertains to photopolymerizable compositions including a benzanthrone component which improves the reproduction quality of photographic elements using the composition. These elements are useful as lithographic printing plates.
2. Description of the Prior Art
It is well known that lithographic printing plates may be formed by coating a photopolymerizable composition onto a substrate such as a hydrophilic aluminum or treated aluminum substrate. Exemplary photopolymerizable compositions are disclosed in U.S. Pat. Nos. 5,061,605 and 5,286,594 which are incorporated herein by reference.
A photopolymerizable composition typically contains a radical generator component such as a photoinitiator, a photopolymerizable monomer or oligomer as well as binders, colorants and other art recognized ingredients. Upon exposure to ultraviolet or actinic radiation through a photomask, the photoinitiator generates free radicals which initiates the polymerization of the monomer or oligomer in the light struck image areas. Unexposed coating areas are then removed by a developing solution resulting in an image of exposed, oleophilic photopolymer against a background of a hydrophilic substrate.
One problem with such plates is out of contact exposure in the intended nonimage areas. The photomask is employed to differentiate between the areas which are intended to be exposed to radiation and those which are to be masked from such exposure. However, the masked areas are often in the form of minute halftone dots of varying size. In practice, there tends to be some minor degree of unwanted exposure in the nonimage areas due to light scattering, diffusion, imperfect mask to plate contact, and other diverse reasons. This causes an unwanted partial exposure in the nonimage areas and hence a background staining due to halations and hotspots. Halations and hotspots are a serious problem in commercial printing applications since they cause degradation of the reproduction quality of halftone images. Those skilled in the art have attempted a variety of solutions to this problem including the selection of photopolymerizable compositions requiring high exposure doses and the use of exposure filters. Such have well known disadvantages. It would be desirable to produce a photopolymerizable composition which can more adequately differentiate between the image and non-image areas by permitting full effective photopolymerization in the unmasked image areas intended to be exposed to actinic radiation while impeding photopolymerization in the masked nonimage areas which are not intended to be exposed. The present invention achieves this purpose by inclusion of a small amount of benzanthrone or a benzanthrone derivative within the photopolymerizable composition. The benzanthrone acts as a polymerization gate or switch. In the unmasked areas which are to receive a full measure of exposure beyond a certain threshold level, photopolymerization proceeds effectively as if no benzanthrone were present. However, in areas receiving a partial exposure to such radiation, i.e. below a threshold level, via light scattering or flare, the benzanthrone impedes unwanted polymerization. Without being limited to a particular theory, it is believed that the benzanthrone acts to prevent or limit polymerization in the masked areas where it receives relatively low exposure doses, but is essentially ineffective for this purpose and is overwhelmed by large exposure doses in the unmasked areas. The result is a photographic element having improved image-nonimage differentiation. Reproduction quality is improved, hotspots and halations caused by out of contact exposure are reduced without reduction in the extent of desired photopolymerization. In photopolymerizable compositions to which small quantities of benzanthrone have been added, exposure to actinic radiation below a threshold level results in substantially no polymerization. However, when exposure exceeds the threshold level, photopolymerization proceeds as if no benzanthrone were present. While it is known from U.S. Pat. Nos. 4,367,280 and 4,386,153 to use benzanthrones as photoinitiators, they have not been known heretofore known as photopolymerization gates in compositions using other components as photoinitiators.